Flexographic printing is a method of direct rotary printing that uses resilient relief image plates. The plates are typically made of rubber or photopolymer. Flexographic printing has found particular application in packaging where it has displaced rotogravure and offset lithography printing techniques in many cases. While flexographic printing can produce high quality printed products, making flexographic printing formes according to prior art processes can be undesirably time consuming and labor intensive.
Typical conventional flexographic plates have a flat polyester base coated with a photopolymer layer. The photopolymer layer is sensitive to ultraviolet (UV) radiation, such that it hardens when exposed to UV light. In a first step, a floor is set by exposing the back of the plate to UV light. The floor forms the base of the relief that will be formed in further imaging steps. A film mask, which is imaged in a separate process, is placed over the top of the photopolymer layer, and drawn down by a vacuum frame to ensure good contact. The photopolymer layer is then flood exposed to UV light, thereby hardening or cross-linking the regions of the photopolymer layer not covered by the mask. The mask is then removed and the plate is processed in solvents to remove the unexposed areas of the photopolymer layer (which were covered by the mask) thus forming a printing image. After processing with solvents the plate is dried. Drying may take several hours.
Digital flexography follows a similar process except that the plate has an integral UV-opaque mask layer coated over the photopolymer layer. The mask layer is selectively ablated by a digital imager with a high-power laser imaging head to form an image mask that is opaque to UV light in non-ablated areas. Once the mask is formed, processing of the plate continues as it would for conventional flexographic plates except that there is no need to use a vacuum frame to ensure good contact between the mask and photopolymer layer since the mask layer is integral with the photopolymer layer. Other flexographic plate formulations, such as Cyrel® Fast made by E. I. Dupont de Nemours and Company, eliminate the use of solvents for the processing step and reduce the combined processing and drying time.
The processed flexographic plate is then mounted on a flexographic press cylinder using an adhesive layer such as a double sided adhesive tape or foam. The imaged plate must be mounted in precise registration on the cylinder. This is done using mechanical and/or electronic aids. Accurate registration is key in producing a high quality printed product. Mounting is typically done by skilled operators.
When a rectangular flexographic plate is mounted on a press cylinder there is a gap or “seam” where the top and bottom of the plate approach one another. On the printing press, the printing stock is backed up by an impression cylinder. The impression cylinder presses the printing stock into contact with the flexographic relief plate mounted on the press cylinder. The impression cylinder sets a contact pressure for the printing operation. Since the seam contacts the impression cylinder on each rotation, the discontinuity jolts the impression cylinder slightly, a phenomenon known as “press bounce” or “cylinder bounce”. This jolt puts an upper limit on the impression speed, beyond which registration and other printing errors may occur.
A common method of reducing the effects of cylinder bounce is to stagger the seam around the cylinder. This method is particularly effective when a repeated pattern is imaged across the cylinder; a common situation in flexographic printing. The plates are arranged so that the impression cylinder is always contacting a relief image and does not fall into a seam. A staggered seam can be achieved by laying out the image so that several plate sections are applied to the cylinder in what are known as “lanes”. In FIG. 1-A a number of plate sections 40 have been cut and imaged and in FIG. 1-B plate sections 40 are shown wrapped around a cylinder 32. Each seam 42 is offset from the seams of other lanes so that they are distributed around the circumference of the cylinder. Consequently the impression cylinder no longer falls into a seam since it is always riding on the image relief of one or more lanes.
A staggered seam may also be achieved by cutting the plate seam in a staircase shape. FIG. 1-C shows a photopolymer plate 30 cut with a staircase seam 33. The seam layout has the same repeat as the image elements 31. In FIG. 1-D plate 30 is shown wrapped around cylinder 32. The location of seam 33 is chosen so that the plate completely wraps around the cylinder with the seams precisely lining up.
While a staggered seam is effective in reducing the effects of cylinder bounce, the manual cutting, mounting, and registration of the plates on the press cylinder is both time consuming and may not provide the accuracy required for high quality printing.
To avoid registration problems, the mask layer may be imaged after mounting the plate on the cylinder. In this way, the registration is provided by the imaging device, which can place an image very accurately. The UV exposure and processing of a plate imaged while on a cylinder in this manner requires specialized equipment, now commonly available, that can operate on round cylinders rather than flat plates.
In order to make the handling of cylindrical photopolymer plates more convenient, sleeve substrates have been developed. A sleeve substrate typically comprises a cylindrical tube of nickel, polyester or some other material. The sleeve substrate material is chosen to have a certain degree of elasticity so that air pressure can be used to expand the sleeve slightly, thus allowing it to be slid over a cylinder on a cushion of air. Once the air supply is removed, the sleeve substrate shrinks so that it is held tightly in place. A photopolymer plate, referred to herein as a “flexographic printing precursor”, may be mounted on the sleeve substrate using double-sided tape in the same way flat plates are mounted on a cylinder. The cut photopolymer plate is wrapped around the sleeve in approximate registration and then imaged on a digital imager to produce a flexographic printing forme, which is then ready to be placed on a printing cylinder for use in a flexographic printing operation. This process employing a sleeve substrate as a base for mounting a flat plate is known in the industry as Plate-on-Sleeve (PoS).
FIG. 2 shows a flow diagram of a prior art process for making a typical PoS flexographic printing forme. A flexographic printing precursor 1 comprising a photoplymer layer and a UV opaque mask layer is back exposed in step 2 to set a floor for the relief image. In step 3 the flexographic printing precursor is cut into sections so that it can be applied to a sleeve substrate in lanes to form a staggered seam. The flexographic printing precursor sections are then mounted on a sleeve substrate using double-sided tape in step 4 to produce a flexographic printing sleeve. Registration of the flexographic printing precursor sections must be accurate enough to ensure that the image will not run into a seam, but since the flexographic printing sleeve is not yet imaged, the accuracy required is significantly reduced. Alternatively, the flexographic printing precursor may be cut to form a section with a staggered seam as shown in FIG. 1-C and mounted as a single piece to a sleeve substrate in step 4.
Referring again to FIG. 2 image data 7 is typically pre-formatted by one or more computer workstations connected to a network to enable file or data transfer. A packaging workflow system 5 and a controller 6 combine to layout an image including the details of how it will be imaged and printed. These workstations provide functionality enabling an operator to take an image file from a customer and arrange the image for optimal printing.
The UV opaque mask layer is then ablated in a digital imager 8 according to the image data 7. The flexographic printing sleeve is then exposed to UV light in step 9, hardening or cross-linking areas where the UV opaque mask layer has been ablated. A processing step 10 follows. Processing may include washing in solvents, drying, and a final UV exposure to fully harden the photopolymer and remove tackiness. The finished photopolymer printing forme 11 is then ready for printing on a flexographic press.
Direct engraving of flexographic plates is also known in the art. Typically a high power laser is used to remove the unwanted material thus forming a relief image. In U.S. Pat. No. 5,416,298 to Robert an apparatus for preparing a printing medium for use in a printing process uses a laser beam to directly engrave the medium. The printing medium may include a printing cylinder for a flexographic printing process. The patent describes an acousto-optic modulator for deflecting the beam over the surface of the medium being engraved.
Digital imaging devices for imaging such flexographic printing sleeves are typically built in the general form of a lathe. Such machines have a mandrel on which a flexographic printing sleeve can be mounted, a fixed headstock for driving the flexographic printing sleeve, a moveable tailstock for supporting the flexographic printing sleeve, and a traveling imaging head. The imaging head typically has a radiation source, such as a laser, capable of imagewise ablating the mask layer.
It is desirable that the overall time required to make a flexographic printing forme be reduced.